In an optical communication field, as an optical circuit which can easily attain the branch and interference of light, an integrated optical part using an optical waveguide structure has been developed. In the integrated optical part using the characteristic as the wave of the light, an optical waveguide length is adjusted to make the manufacturing of an optical interferometer easier, or a circuit processing technique in a semiconductor field is applied, which makes the integration of the optical parts easier.
Such an optical waveguide structure is an “optical confinement structure” where with regard to the light propagating through the optical waveguide, the spatial distribution of refractive indexes is used to attain a spatial optical confinement. In order to constitute the optical circuit, an optical wiring and the like are used to connect respective components in cascade arrangement. For this reason, an optical path length of the optical waveguide circuit must be longer than an optical path length required to generate an interference phenomenon and the like inside the optical circuit. This results in a problem that the optical circuit itself is extremely large in scale.
For example, when a typical arrayed waveguide grating is exemplified, a plurality of lights having a wavelength (λj) inputted from an input port are repeatedly branched and coupled by a star coupler having a slab waveguide. Then, the branched lights are outputted from an output port. However, an optical path length required to branch the light at a resolution of about 1/1000 of a wavelength becomes several ten thousand times of a wavelength of the light propagating through the waveguide. Also, not only the waveguide patterning of the optical circuit, but also the process for installing a wavelength plate and the like to compensate the circuit property depending on a polarized light state is required to be performed (for example, refer to a Non-patent Document 1).
Also, in order to miniaturize the optical circuit, the light is required to be strongly confined in the waveguide. Thus, the optical waveguide is required to have a very great refractive index difference. For example, in the optical waveguide of a conventional step index type, the optical waveguide is designed to have the spatial distribution of the refractive indexes so that a specific refractive index difference has a value greater than 0.1%. When such a great refractive index difference is used to carry out the optical confinement, the degree of freedom of the circuit configuration is limited. In particular, even if the refractive index difference in the optical waveguide is attempted to be attained by using a local UV light irradiation, a thermo-optic effect or an electro-optic effect or the like, the change amount in the obtained refractive index is about 0.1% at most.
Moreover, in a case of changing the propagation direction of the light, when the optical path of the optical waveguide is curved at a small curvature, the propagating light is leaked out from the optical waveguide. Thus, the circuit property is deteriorated such as an increase in a transmission loss and the like. Therefore, in order to change the propagation direction of the light, the orientation must be gradually changed along the optical path of the optical waveguide. Inevitably, the optical circuit length becomes very long. As a result, it is difficult to miniaturize the optical circuit.
Therefore, the optical circuit that is high in efficiency and small in size is attained by using a wave propagation medium which is smaller than the optical circuit using the conventional optical waveguide circuit and holographic circuit and enables the optical signal control of a sufficiently high efficiency under a gradual refractive index distribution, namely, even under a small refractive index difference.
However, in the wave propagation medium, in accordance with the refractive index of each virtual pixel defined by a virtual mesh, an optical signal, while undergoing multiple scattering, is transmitted from the input port to the output port. Thus, a manufacturing error when the mesh-shaped pixel is generated causes the leakage of the transmission light. Consequently, the interference is generated between the propagation light outputted to the output port and the leaked transmission light of the same wavelength, and even if the wavelength is different and the interference is not generated, crosstalk is generated.
Also, the wave propagation medium transmits an optical signal by using the effect of the interference. Thus, a large angle cannot be given to the optical path, and crosstalk is great. Since the effect of the interference is different depending on the angle of the incident light, the fact that the rate for the oblique incident component is high leads to one reason of the deterioration in crosstalk. In particular, in a region where a beam diameter of the light near the input and output ports is small, the rate of the component obliquely incident with respect to the propagation direction of the light is great, which deteriorates the crosstalk. There is a problem that the circuit property is further deteriorated such that the transmission loss of the optical circuit becomes greater because of the deterioration of the crosstalk as mentioned above.
Moreover, the mesh-shaped pixel near the input port and the output port in the wave propagation medium can function as a kind of lens and collect the lights. However, because of the mesh-shaped pixel, there is a problem that the discrimination of a light focusing position is difficult and the connection to a different optical element is difficult.
Non-patent Document 1: Y. Hibino, “Passive Optical Devices for Photonic Networks”, IEIC' Trans. Commun., Vol. E83-B No. 10, (2000).